This invention relates to an improved type of coating intended for constituting the active surface of a metal electrode of use in the electrolysis of alkali metal halides, and, particularly, in the production of sodium chlorate from said electrolysis.
In electrolytic cells for the production of chlorine, such as those of the diaphragm and membrane type, an aqueous solution of an alkali metal halide is electrolyzed to produce chlorine at the anode and an alkali hydroxide and hydrogen at the cathode. The products of electrolysis are maintained separate. In the production of sodium chlorate, the chlorine and alkali hydroxide are allowed to mix at almost neutral pH and the sodium chlorate is formed via disproportionation of the sodium hypochlorite formed in the above mixing.
U.S. Pat. No. 3,849,282--Deguldre et al., describes a coating for metal electrodes, which coating comprises a compound ABO.sub.4 having a rutile-type structure, where A is an element in the trivalent state selected from the group rhodium, aluminum, gallium, lanthanum and the rare earths, while B is an element in the pentavalent state selected from the group antimony, niobium and tantalum, the compound ABO.sub.4 being associated with an oxide of the type MO.sub.2 where M is ruthenium and/or iridium. The electrodes described therein may be used in various electrochemical processes such as cathodic protection, desalination or purification of water, electrolysis of water or hydrochloric acid, production of current in a fuel cell, reduction or oxidation of organic compounds for the electrolytic manufacture of per salts, and as anodes in the electrolysis of aqueous solutions of alkali metal halides, particularly sodium chloride, in diaphragm cells, mercury cells, membrane cells and chlorate production cells, where they catalyze the discharge of chloride ions. The electrodes described therein are stated to adhere to their metal support and are stated to be resistant to electrochemical attack.
U.S. Pat. No. 3,718,551--Martinsons, describes an electroconductive coating for metal electrodes, which coating comprises a mixture of amorphous titanium dioxide and a member of the group consisting of ruthenium and ruthenium dioxide. The electrodes described therein are characterized by having a low oxygen and chlorine overvoltage, resistance to corrosion and decomposition for coatings containing less than 60% by weight of titanium (as oxide) based on the total metal content of the coatings.
Neither U.S. Pat. No. 3,718,551 or 3,849,282 gives any teaching on the current efficiency of the electrodes for the oxidation of chloride in aqueous solution. Kotowski and Busse, Modern Chlor Alkali Technology, Volume 3, page 321, comment on the relationship between overvoltage and oxygen evolution for the oxidation of aqueous chloride solutions using coatings of the type taught by U.S. Pat. No. 3,718,551 wherein a linear relationship between overpotential and log oxygen content in chlorine (increasing one - reducing the other) is given. Moreover, increasing ruthenium content is stated to result in increased oxygen evolution and reduced overpotential.